Modulation meter



July 11, 1939. w. BUSCHBECK MODULAT ION METER Filed July 24. 1936 RECT/F/ER W4 VE INVENTOR WERN ER BUSCH BECK BY gg ATTORNEY Patented July 11, 1939 '1 a I UNITED- PATENT OFFICE 2,165,836- MODULATION METER Werner Buschbeck, Berlin, Germany, assignor to,

'Ielefunken; Gesellschaft fiir Drahtlose Tole:- graphie; m. b. H., Berlin, Germany, a corporation of Germany Application July 24, 1936, SerialNoc 92,304

In Germany August 14,1935

8 Claims. (01. 179-171 Hithertodifliculties have been encountered in An example of: an embodiment of the idea of manufacturing a control meter, to be used with the inventiontis shown in Figure 1. The antenna modulated-transmitters arranged 'to-cover a wide IB- or the intermediate. circuit of a modulated wave range or to use different power stages, in transmitter has a detector I coupled thereto. The

other words for transmitters having variable condenser 2 isa short circuitwith respect to the 5 amplitude currents, which does not require adhigh frequency wave energy, but 2 is of extremely justment. As is known, the degree of modulation high impedance with respect to the demodulated is defined by the ratio between the low frequency low frequency. Then at the combined resistors alternating voltage amplitude attained at an 3 and there appears a direct current potential ohmic resistor following linear detection, and the corresponding to the amplitude of the carrier to drop of the direct voltage corresponding to the wave of the transmitter as well as the low frecar-rier wave amplitude or intensity. In the orquency alternating potential corresponding to the dinary control meter circuits the low frequency degree of modulation. If for instance the resistor measuring instrument is ordinari-ly gauged in-per- 3 is an ordinary resistor the value of which is in- 5' cent of degree of modulation, but this calibration dependent of the current amplitude and intensity, can only be correct for a definite direct current while the resistor 4 is a hot conductor with a charintensity, in other Words for high frequency curacteristic as shown forinstance in Figure 2, rent of a definite amplitude at a given coupling namely such that the voltage across the resistor between the meterand the high'frequency circuit. 4 ispractically-i pendent of the value of the If the high frequency current amplitude changes direct current within the control limits of the hot 20 (or the coupling) at changing of the waveto be conductor, in other words, independent of the monitored, or in case a different number of or high frequency r nt of the modulated' an type of power stages-are used, the coupling ofthe mitter. Since the hot conductor 4, owing to its detector to the transmitter must be subsequently heat capa y has a building p period Of Se adjusted; seconds up to approximately minute (accord- To avoid this difficulty, circuitshave beenproing to typ n r si n e variat n takes place posed already and have been constructed which in at the rhythm of the demodulated low frequency,

fact permit the measurement of the degree of accordingly, no component characteristic ofsaid modulation, 1. e., the ratio between the low frelow frequency can be derived from the resistor 4 quency and the direct current component followin a known manner and applied to an instrument to ing detection over a range of frequencies and for mean value or peak value. amplitude. In this connection, reference is made, The manner of operation of the disclosed defor instance, to the cross coil instruments with Vice is based up n h ac that e ed high variable air gap which afford a direct indication frequency oscillation tobe investigated is led to an of the modulation ratio. Suchinstruments are ordinary re n t 110 a resistance of the 35. expensive and cumbersome, because these cross constant volta e V rying current type which are coil instruments are comparatively sensitive, and connected in series, Resistors 3 and 4 are so confurthermore because the low frequency compon cted in Fi ure 1. The non-linear resistance t is nent requires an additional detection. of a material such that the direct component of In this application, a control meter will be dethevoltage occurringinit remains constant when 40 scribed which while using ordinary instruments the voltage across the terminals of the whole permits direct calibration in percent of degree of series connection is slowly varied. This is due to modulation, despite variable input voltages at the ac t at On a Variation of the total Voltage the detector which vary by a ratio of 1:5. To applied to the series connection, the ratio of the 5 complish this object in accordance with the involtage division between resistances 3 and d is vention, self controlling resistors maintaining the altered, and this occurs in such a way that on voltage or current constant of the type-known as increasing the total voltage the resistance of cup o hydrogen or i d of uranium i t rent dependent resistance 4 is. decreasing so that are employed. the partial voltage thereat remains constant.

In describing my invention reference will be Now, if at t terminals of the series connection 50 made to the attached drawing wherein Figures a di ect Voltage s perpose by alternating volt- 1 and 3 show two embodiments of my meter, age (rectified carrier and modulation oscillation) while Figure 2 is a curve showing the current is applied, then the resistance of the current devoltage characteristic of an impedance used: in pendent resistance due to its inertia, does not vary thecircuits of Figures 1 and 3. c in accordancewiththe alternating current. This 55 alternating current can be taken from the resistance 4 and appears in the meter. The magnitude of the current dependent resistance, on the contrary, depends only on the magnitude of the direct voltage. If, therefore, the direct voltage be increased the alternating voltage remaining constant (small degree of modulation) the resistance of the current dependent resistance 4 will decrease. That is to say, the amplitude of alternating voltage taken off the current dependent re-' sistance will also become smaller, which is directly corresponding to the degree of modulation. If the direct voltage and simultaneously the alternating voltage (constant degree of modulation) be increased, the resistance of current dependent resistance 4 will decrease, the alternating voltage taken from 4 will remain constant, however, as the alternating voltage applied is just. as much bigger as the current dependent resistance has decreased.

In the final case possible, that the direct current decreases while the alternating current remains constant (high degree of modulation) the magnitude of the resistance of current dependent resistance 4 will increase so that the alternating voltage taken from 4 is big corresponding to the degree of modulation. It is to be seen, therefore, that the alternating voltage taken off the current dependent resistance is really in direct proportion to the degree of modulation, and this proportion is independent of the absolute value of the carrier oscillation and, therefore, independent of the degree of coupling between the measuring circuit and the circuit to be measured. In other words, for a given signal the ratio of the alternating voltage across 4 to the direct voltage across 4 remains the same for all couplings between 10 and 1.

Strictly speaking, the indication would only be absolutely correct if the control resistor would be passed by direct current only, and not by direct current and low frequency current together as in the arrangement shown. However, in prac tice this condition is immaterial. It was found from oscillographic records of transmitters modulated by speech or music, that at maximum degree of modulation such as 90% and more, the average degree of modulation does not exceed the value of 25 to 30%, since the high modulation peaks last only a very short time as known by experience. Owing to its large heat inertia, the hot conductor integrates the varying current pulses over a longer time period, so that the effective current can actually be calculated from the average degree of modulation. It is known that wherein JTr is the amplitude of the carrier wave, and K the degree of modulation. At an average modulation of 28% therefore, there is:

JTr

of Figure 1, item 3 is to be replaced by the iron hydrogen resistor of the constant current type, and item 4 is to be replaced by an ordinary resistor. Otherwise the same considerations are true as stated above. This modification is shown in Figure 3 of the drawing.

When instead of constant voltage varying current resistances, varying voltage constant current resistances are used, the taking off of the measuring alternating current has to take place at the normal resistance 3 as shown in Figure 3. The constant current varying voltage resistance 4 is in series with resistance 3, as in the arrangement of Figure 1. In other respects, how ever, the manner of operation is exactly the same as described hereinbefore.

If very large current variations are to be expected in the coupling circuit such as for instance at power control within wide limits, it will be advisable to adapt to a certain extent the working condition of the high frequency detector to the varied operating conditions in order to remain within the control range of the self controlling resistors. This may be accomplished for instance in that in common with the operating organ for the power control there is also varied in a continuous fashion or in steps the coupling of the detector or the constant resistor connected in series to the self controlling resistor.

I claim:

1. In a system for direct indication of the degree of modulation of modulated high frequency current of variable mean amplitude, a rectifier having output terminals, means for impressing said high frequency current on said rectifier, a resistor having a non-linear direct current-voltage characteristic, a second resistor having a substantially linear direct current-voltage characteristic, means connecting said resistances in a series circuit, means connecting said series connection across the output terminals of said rectifier whereby rectified energy flows in said resistances and voltages are produced across said resistances by said direct current flowing therein, and an alternating current measuring instrument coupled in shunt to one of said resistances.

2. A system as recited in claim 1 wherein said non-linear resistance is of the dioxide of uranium type.

3. A system as recited in claim 1 wherein said non-linear resistance is of the iron hydrogen type.

4. In a modulation meter adapted to indicate the degree of modulation of carrier waves of different amplitude, a rectifier having input and output electrodes, an alternating current circuit which may be energized by modulated wave energy coupled to said input electrodes, an impedance having a substantially constant voltageampere characteristic and an impedance having a non-linear voltage-ampere characteristic coupled to the output electrodes of said rectifier, and indicating means connected with one of said impedances.

5. A meter as recited in claim 4 wherein said impedance having the non-linear voltage-ampere characteristic is of the constant current type.

6. A meter as recited in claim 4 wherein said impedance having the non-linear voltage-ampere characteristic is of the constant voltage type.

'7. In a modulation meter adapted to indicate degree of modulation of carrier waves of diiferent amplitude, a rectifier having input and output electrodes, an alternating current circuit which may be energized by modulated wave energy coupled to said input electrodes, at substantially linear and a non-linear impedance coupled to the output electrodes of said rectifier, said linear impedance having a linear direct current-voltage characteristic, said non-linear impedance having a constant direct current-variable voltage characteristic, and indicating means connected in shunt to said substantially linear impedance.

8. In a modulation meter adapted to indicate the degree of modulation of carrier waves of different amplitude, a rectifier having input and output electrodes, an alternating current circuit which may be energized by modulated wave energy coupled to said input electrodes, a substantially linear and a non-linear impedance coupled to the output electrodes of said rectifier, said. linear impedance having a linear current-voltage characteristic, said non-linear impedance being of the constant voltage variable current type, and indicating means connected in shunt to said nonlinear impedance.

WERNER BUSCHBECK. 

